Dynamics of femtosecond-laser-induced lateral motion of an adsorbate: O on vicinal Pt(111)

For the system of oxygen adsorbed at the step sites of a vicinal Pt(111) surface it has been demonstrated that diffusion of strongly bound atomic adsorbates can be induced electronically with femtosecond laser pulses (Stepan et al 2005 Phys. Rev. Lett. 94 236103). The dynamics of energy transfer between the initial electronic excitation of the metallic substrate and the adsorbate motion is examined by comparing experimental hopping rates with those obtained from different model calculations. For this purpose the adsorbate–substrate coupling is approximated by an electronic friction. The experiment shows a strong nonlinear dependence of the hopping rate on laser fluence (F15) and a coupling time of 1.5 ps between electronic excitation and diffusive motion as deduced by two-pulse correlation measurements. The consistent modelling of these findings requires the introduction of a friction coefficient that strongly depends on electron temperature. This result is interpreted in terms of an indirect excitation mechanism. It is proposed that anharmonic coupling between primarily excited perpendicular O–Pt vibrations and frustrated translations results in an effective coupling strength that increases with increasing electron temperature.